The present invention relates to an improved process for the preparation of 2,3,5,6-tetramethyl-1,4-diisocyanatobenzene (hereinafter referred to as xe2x80x9cdurene diisocyanatexe2x80x9d) by (1) reaction of 1,2,4,5-tetramethyl-benzene (hereinafter referred to as xe2x80x9cdurenexe2x80x9d) with nitric acid in sulfuric acid, (2) catalytic hydrogenation of the resultant 2,3,5,6-tetramethyl-1,4-dinitrobenzene (hereinafter referred to as xe2x80x9cdinitrodurenexe2x80x9d), and (3) phosgenation of the resultant 2,3,5,6-tetramethyl-1,4-diaminobenzene (hereinafter referred to as xe2x80x9cdurene diaminexe2x80x9d).
The preparation of durene diisocyanate is known. One of the known processes is described in British Patent Specification 779,806. In accordance with British Patent 779,806, durene diamine is dissolved in chlorobenzene, then reacted with gaseous hydrogen chloride to form durene diamine dihydrochloride, and the latter is further reacted as a suspension in chlorobenzene with phosgene at elevated temperature.
Durene diamine has been known for a long time and is normally prepared from dinitrodurene by reduction. The hydrogenation of dinitrodurene to durene diamine in ethanol with the addition of Raney nickel as catalyst is described, for example, in the Journal of the American Chemical Society, Vol. 70, p. 2227 and Vol. 72, p. 132.
Dinitrodurene has similarly been known for a long time and is normally prepared by nitration of durene. An improved process for the preparation of dinitrodurene is described in U.S. Pat. No. 3,153,099. In accordance with U.S. Pat. No. 3,153,099, a suspension of durene inconcentrated aqueous sulfuric acid is reacted with a nitric acid-sulfuric acid-water mixture in the absence of organic solvents at temperatures from 5xc2x0 to 10xc2x0 C., the resultant suspension is then stirred into an ice-water mixture and the dinitrodurene is filtered off, washed with water, and dried at 60xc2x0 to 70xc2x0 C.
U.S. Pat. No. 3,153,099 teaches, the obvious procedure of dissolving durene in an organic solvent and subjecting this solution to a nitration has a considerable number of disadvantages. The most significant of these disadvantages are the small yields of dinitrodurene and the large amounts of by-products. These disadvantages are illustrated in Example 1 (comparison example) of the present patent specification.
Admittedly, the nitration of durene in sulfuric acid suspension represents an improvement over the nitration of a solution in an organic solvent. However, the process described in U.S. Pat. No. 3,153,099 has serious disadvantages. For example, the reaction temperature range of 5xc2x0 to 10xc2x0 C., which is said to be preferred, results in uneconomically long reaction times. This is also confirmed by Example 2 (comparison example) of the present patent specification.
A further disadvantage of the process disclosed in U.S. Pat. No. 3,153,099 is that the end reaction mixture is stirred into an ice-water mixture. Although this use of ice to dilute concentrated sulfuric acid with water in order to control the heat of mixing that is generated is a conventional and practicable method for laboratory scale operations, it is unsuitable for large-scale production processes. This method requires the availability and handling of large amounts of ice, coupled with the considerable technical effort and expenditure involved in producing and transporting the ice. This method also requires processing the ice in large capacity containers, optionally using comparatively powerful stirrer motors. One of the consequences of this process step is that all of the sulfuric acid used for the reaction becomes diluted and must either be disposed of as a valueless waste acid or be reconcentrated (a complicated and expensive process).
A further disadvantage of the process described in U.S. Pat. No. 3,153,099 is that the crude dinitrodurene filtered off is purified only by washing with water. Consequently, impurities such as sulfuric acid trapped in the dinitrodurene particles are not separated. On the other hand, a minimum quality of the dinitrodurene is necessary, especially if the dinitrodurene is to be used to produce durene diamine by reduction as taught at column 3, lines 47 to 50. This generally means that the dinitrodurene that is prepared must be purified in a further process step, for example, by recrystallization.
Only catalytic hydrogenation is technically feasible as a commercial process for the reduction of dinitrodurene to durene diamine. Although other known processes, such as the conversion of dinitrodurene with ammonium sulfide, with zinc or with tin (II) chloride are suitable for conversions on a laboratory scale, for economic and ecological reasons they are unsuitable for large-scale preparation of durene diamine.
The known processes for catalytic hydrogenation in a protic solvent such as ethanol have the disadvantage as regards the conversion of durene diamine with phosgene that the solvent must be substantially completely removed from the reaction mixture. If the solvent is not substantially removed, undesirable reaction products of the solvent with phosgene will be formed during the phosgenation. This means that the solution of durene diamine must be completely evaporated in a complicated process step and the remaining solid durene diamine then must be re-dissolved in an aprotic solvent.
The process described in British Patent 779,806 for the preparation of durene diisocyanate by reacting durene diamine dihydrochloride with phosgene is also affected by serious disadvantages. For example, preparation of durene diamine dihydrochloride using gaseous hydrogen chloride is costly and time-consuming. Likewise, the reaction of the dihydrochloride with phosgene in suspension is laborious and also results in contamination of the process waste gas with comparatively large amounts of hydrogen chloride.
It is an object of the present invention to provide a technical process for the preparation of durene diisocyanate.
It is also an object of the present invention to provide a process for the production of durene diisocyanate in higher yields without uneconomically long reaction times.
It is a further object of the present invention to provide a process for the production of durene diisocyanate without generating large quantities of waste and by-products.
It is an additional object of the present invention to provide a process for the production of durene diisocyanate that does not require considerable technical effort and expenditure.
It is a further object of the present invention to provide a process for the production of durene diisocyanate in which the individual reaction steps and process stages are matched to one another.
These and other objects which will be apparent to those skilled in the art are accomplished by (1) reacting durene with nitric acid in the presence of sulfuric acid. (2) diluting the resultant reaction mixture with water to form a suspension; (3) mixing the resultant suspension with an aprotic organic solvent to dissolve the dinitrodurene and to form two liquid phases; (4) separating the liquid phases to recover a solution of dinitrodurene in organic solvent; (5) hydrogenating the dinitrodurene in the presence of a catalyst; (6) removing water and catalyst from the hydrogenated mixture; and (7) phosgenating the diaminodurene from (5).
The present invention is an improved process for the preparation of durene diisocyanate by reaction of durene with nitric acid in sulfuric acid, catalytic hydrogenation of the dinitrodurene that is formed, and phosgenation of the resultant durene diamine. In this process, after the reaction of durene with nitric acid in sulfuric acid has been completed, the liquid phase of the reaction mixture, which is composed substantially of sulfuric acid, is diluted by mixing it with water. The resultant suspension is intensively mixed with an aprotic organic solvent that is substantially immiscible with water and is inert with respect to hydrogen to dissolve the dinitrodurene in the solvent and form a mixture composed of two liquid phases. The solution of dinitrodurene in the solvent is separated from this mixture by phase separation and then optionally purified by extraction with water. The dinitrodurene solution thus obtained is subjected to hydrogenation in the presence of a solid, insoluble catalyst, optionally after concentration by distillative separation of solvents. The water and the catalyst are separated from the hydrogenated reaction mixture and the remaining solution of the resultant durene diamine is then phosgenated.
It was extremely surprising that the process of the present invention produces such high yields of durene diisocyanate, even though the intermediate products dinitrodurene and durene diamine are not isolated and separately purified. What is particularly surprising is the fact that dinitrodurene can be separated in such a state of purity from the reaction mixture formed by the reaction of durene with nitric acid merely by dissolution in an aprotic solvent and that the subsequent catalytic hydrogenation can be successfully carried out.
In the process of the present invention, the reaction of durene with nitric acid in sulfuric acid is largely carried out in accordance with the procedure disclosed in U.S. Pat. No. 3,153,099 with the exception that no working-up of the product dinitrodurene is conducted in the process of the present invention.
In the process of the present invention, durene is used in finely divided form in order to achieve as complete a conversion as possible in suspension.
In order to prepare the suspension used in the process of the present invention, durene is mixed with from 5 to 20 times, preferably from 7 to 15 times the amount by weight of concentrated sulfuric acid. The concentrated sulfuric acid generally has a water content of from 2 to 40 wt. %, preferably from 4 to 20 wt. %.
Pure nitric acid can be used for the reaction with durene. Mixtures of nitric acid with sulfuric acid and/or water may, however, also be used. The proportion of sulfuric acid in such mixtures may be from 0 to 75 wt. % and the proportion of water may be from 0 to 40 wt. %.
In the nitration reaction, nitric acid is used in amounts of from about 2.0 to about 2.2 moles, preferably from about 2.0 to about 2.1 moles, for each 1 mole of durene.
The reaction of durene with nitric acid is carried out in the process of the present invention at temperatures of up to 15xc2x0 C., preferably at from 10xc2x0 to 15xc2x0 C., most preferably at from 11xc2x0 to 13xc2x0 C. Although reaction temperatures below 10xc2x0 C. are possible in principle, the resultant low reaction velocity increases the danger that the concentration of nitric acid in the reaction mixture will increase. Increased nitric acid concentration could give rise to an uncontrollable exothermic reaction. To minimize this safety risk, reaction temperatures of less than 10xc2x0 C. should generally be avoided.
In one embodiment of the present invention, a substantial proportion of the liquid phase is separated from the nitrated reaction mixture by filtration or centrifugation of the dinitrodurene. The proportion of the spent acid separated in this way is generally from about 50 to about 95 wt. %, preferably from about 60 to about 90 wt. % of the overall liquid phase. The amount of spent acid separated is limited by the efficiency of the filtration or centrifugation process.
In a preferred embodiment of the present invention, the spent acid is reused for a subsequent reaction of durene with nitric acid. To this end, the spent acid is generally mixed with additional amounts of concentrated or aqueous or optionally pure sulfuric acid before the nitration reaction. The concentration and amount of added sulfuric acid depend on the amount and composition of the separated spent acid. In general, however, an attempt is made to use approximately the same weight ratios of durene, sulfuric acid and water for each reaction of durene with nitric acid.
In accordance with the process of the present invention, the end reaction mixture of the reaction of durene with nitric acid is mixed with water. If the process is not carried out according to the special embodiment (i.e., by separating the prepared crude dinitrodurene by filtration or centrifugation before mixing with water), it is generally convenient to add water to the reaction vessel and then stir the reaction mixture into the water. It may, however, also be advantageous to mix the end reaction mixture in a continuous stream with a water stream in a mixing unit and thereby cool the mixture.
If the crude dinitrodurene is separated by filtration or centrifugation before mixing with water, it is convenient to suspend the dinitrodurene filter cakes in the water. If desired, the resultant suspension may at the same time also be conveyed to another vessel by the water flow.
The amount of water used for the mixing depends on how much sulfuric acid is mixed with the crude dinitrodurene. Normally, sufficient water is used so that the resultant liquid contains less than 40 wt. %, preferably less than 30 wt. % of sulfuric acid. In general, it is convenient to use sufficient water so that the temperature of the resultant mixture is not more than 100xc2x0 C. In order to avoid use of an uneconomically large amount of water, it may be advantageous to carry out the mixing with water under efficient external cooling. However, the cooling is generally not necessary if the reaction mixture has been freed (e.g., by filtration or centrifugation) from a substantial proportion of the liquid phase before mixing with water.
In the process of the present invention, the suspension of crude dinitrodurene formed by mixing with water is intensively mixed with an aprotic organic solvent that is substantially immiscible with water. Suitable solvents are those compounds that are inert with respect to aqueous sulfuric acid and durene diisocyanate and are also inert under the conditions of the catalytic hydrogenation and the phosgenation conditions. Examples of suitable solvents include: isooctane, cleaning naphtha, decahydronaphthalene, toluene, m-xylene, 1,2,3,4-tetrahydronaphthalene, chlorobenzene, o-dichlorobenzene, 2-chlorotoluene and 1-chloronaphthalene. Mixtures of suitable solvents may also be used in the process according to the invention. However, toluene or chlorobenzene is preferably used as solvent.
In the present invention, the intensive mixing of the dinitrodurene suspension with the organic solvent may take place at low temperatures. The mixing is, however, conveniently carried out at elevated temperatures in order to accelerate the dissolution process. The temperature is below the boiling point of the relevant solvent and is generally from 30 to 95xc2x0 C. It may be advantageous to adjust the temperature of the aqueous dinitrodurene suspension to about 90 to 95xc2x0 C. by choosing a correspondingly small amount of water for the mixing to achieve the desired elevated temperature on dissolution in the solvent. Obviously it is also possible to raise the temperature by using a heated solvent and/or by external heating during dissolution.
The amount of solvent used to dissolve the dinitrodurene depends on the solvent power of the solvent and on the temperature of the mixture and can easily be determined by appropriate preliminary experiments. It is convenient not to use significantly more solvent than is necessary to dissolve the dinitrodurene. A high dilution with solvent is in general disadvantageous because large capacity vessels are then necessary for preparing and processing the solution. Amounts of solvent in the range from 3 to 100 kg, preferably from 4 to 20 kg for each 1 kg of prepared dinitrodurene, are generally used.
The separation of the dinitrodurene solution from the aqueous phase may be carried out in the process of the present invention by phase separation. The amount of time necessary for a sufficient separation depends to a large extent on the composition of the phases and on the nature of the solvent that is used. In order to achieve as rapid a phase separation as possible, the concentrations of the phases are conveniently chosen so that the densities of the phases will be significantly different.
After separation of the organic phase, it may be advantageous to purify that phase by intensive mixing with water and an additional phase separation. This is particularly advantageous if the organic phase still contains a significant proportion of emulsified aqueous acid due to sluggish phase separation.
Extremely dilute solutions may be used for the hydrogenation of the dinitrodurene. In general, however, it is convenient to hydrogenate solutions that are as concentrated as possible in order to achieve high space-time yields. It may therefore be advantageous to separate part of the solvent from the dinitrodurene solutions by distillation before the hydrogenation. Solutions of dinitrodurene with a dinitrodurene concentration of 8 to 40 wt. % are accordingly advantageous.
In the process of the present invention, the dinitrodurene solution is hydrogenated in the presence of a solid catalyst that is practically insoluble in the solvent. Catalysts that are conventionally used in the art for the catalytic hydrogenation of nitroaromatics are suitable for this purpose. Examples of suitable catalysts include: Raney nickel, Raney nickel-iron and Raney cobalt. Precious metal catalysts such as palladium or platinum may of course in principle also be used, but are in general less suitable for economic reasons and the danger of nuclear hydrogenation.
The amount of catalyst used for the hydrogenation depends on the nature of the catalyst, on the type of solvent, and on the concentration and purity of the dissolved dinitrodurene, and varies within a wide range. The optimum weight ratio may easily be determined by suitable preliminary experiments. In general, amounts of catalyst in the range from 0.1 to 10 kg, for each 100 kg of dinitrodurene to be hydrogenated, are used.
The reaction temperature in the catalytic hydrogenation of the dinitrodurene solution also depends on the type of catalyst that is employed and on the type of solvent that is used, and may range from 50 to 250xc2x0 C.
In the process of the present invention, the catalytic hydrogenation of the dinitrodurene solution may be carried out at a hydrogen pressure of from 1.1 to 200 bar, preferably from 3 to 100 bar.
The end reaction mixture of the catalytic hydrogenation is a multiphase mixture that contains, in addition to the solution of durene diamine in the organic solvent, the catalyst as a solid phase as well as water from the hydrogenation reaction as a second liquid phase. In the present invention, the water and the catalyst may be conveniently completely removed from the reaction mixture before the durene diamine solution is used in the phosgenation.
In a preferred embodiment of the invention, the reaction mixture is freed from water by partial distillation before separating the catalyst. If the hydrogenation is carried out at a temperature above the boiling point of the solvent, it may be advantageous to cool the end reaction mixture by flash evaporation. This releases pressure in the pressurized reactor that is used, cools the mixture, and partially distills the mixture to separate water without additional expenditure of energy. After the pressure compensation, sufficient solvent may be distilled off together with the water until the reaction mixture is practically anhydrous. This can take place directly after the flash evaporation in the pressurized reactor. No additional distillation vessel is required.
The separation of the catalyst from the reaction mixture of the hydrogenation or from the anhydrous durene diamine solution may be carried out by any of the methods conventionally used in the art (e.g., by filtration or centrifugation).
The reaction of the anhydrous and catalyst-free solution of crude durene diamine with phosgene in conventional amounts (ca. 2 to 8 moles, preferably 3 to 6 moles of phosgene for each 1 mole of durene diamine) to form durene diisocyanate may be carried out by conventional cold-hot phosgenation. This means that a solution of phosgene in the organic solvent that was, for example, previously used is first added at a low temperature (from about 0 to 30xc2x0 C.) to the phosgenation reactor, the durene diamine solution is mixed in, optionally while cooling. The resultant suspension is then heated to reflux and the reaction is completed under reflux, optionally with the addition of more phosgene.
In a preferred embodiment of the process of the present invention, the optionally hot durene diamine solution is intensively continuously mixed with a solution of phosgene in the solvent in a mechanically driven mixing apparatus, and the resultant suspension is passed to a downstream reactor where the reaction is brought to completion. Conventional equipment used in the art, for example mixing pumps or mixers equipped with teeth, are suitable as mixing apparatus. Use of a mixing apparatus producing a uniform mixing due to shear forces minimizes the formation of by-products. Further, the reaction mixture does not have to be cooled, but can be heated directly to reflux in the downstream reactor.
After completion of the phosgenation, the reaction mixture is normally worked up by distillative separation of the phosgene that is still present together with part of the solvent. The durene diisocyanate that is formed can then be recovered by crystallization and purified by recrystallization, as described in British Patent Specification 779,806. The diisocyanate may be conveniently recovered by distillation and purified by fractional distillation. Durene diisocyanate can be distilled without decomposition at temperatures below 200xc2x0 C. For this purpose, reduced pressures (e.g., those that can be produced without difficulty on a large industrial scale such as pressures of from 1 to 22 mbar) are necessary.
The distillative working-up of the reaction mixture is more advantageous than working-up by means of crystallization because it is substantially less complicated and produces durene diisocyanate of a higher degree of purity. Also, the solvent is recovered in a pure form and can therefore be used again to dissolve the crude dinitrodurene. Finally, fractional distillation makes it possible to recover any small amount of unreacted durene that may still possibly be contained in the crude product in a pure form suitable for re-use in a subsequent nitration.
The process of the present invention may as a whole or in individual partial steps be carried out continuously or batchwise.
Significant advantages of the process of the present invention over the prior art include:
The intermediate products dinitrodurene and durene diamine are not worked up in a complicated manner as solid products, but in an easier to handle dissolved form.
The intermediate products dinitrodurene and durene diamine are not isolated and purified in a complicated manner, but are processed directly after their preparation.
Water is employed for diluting the sulfuric acid and not the more difficult to handle ice used in the prior art.
Use of only one organic solvent for the overall preparation process reduces the amount of equipment and apparatus such as storage vessels and distillation columns necessary to carry out the process.
The process is substantially less environmentally harmful due to the separation and re-use of spent acid in the reaction of durene with nitric acid.
The durene diisocyanate prepared in accordance with the present invention represents a valuable starting product for the preparation of polyurethane plastics.